1. Field of the Invention
The disclosures herein relate generally to location determining systems, and more specifically, to systems that determine location using radio signals.
2. Background of the Invention
Numerous systems have been developed throughout the years to determine location using radio signals. Some of these techniques include triangulation, radio ranging from a collection of direction finding measurements, passive and active tag ranging, time difference of arrival (TDOA) systems and global positioning systems (GPS), for example. These techniques rely on measurements in the “far field” of a radio transmitter. In other words, these measurements are performed at a substantial distance away from the radio transmitter in terms of radio signal wavelength.
The indoor and urban environment exert a substantial impact on the propagation of radio frequency signals. This complex environment is known to effectively scramble or even block the propagation of radio frequency signals, especially those in the VHF, UHF and microwave part of the radio spectrum. Rapid variations in amplitude and phase may occur as these signals scatter and interfere with one another in complex ways. Difficulties with multi-path radio signal reception are commonplace in this hostile environment.
For example, a network of sensors may be deployed throughout an area in which one desires to track personnel or assets. Signal strength measurements may localize a transmitter to a particular zone; however, positioning based on relative signal strength is inherently imprecise.
“RF fingerprinting” is one approach that a radio location system may use to overcome the complicated signal propagation of the urban environment. The motivation behind RF fingerprinting techniques is the hope that a sufficiently accurate map can be made to uniquely identify different transmit positions in the same way human fingerprints serve to uniquely identify different persons. One variation of the RF fingerprinting approach attempts to use multi-path signals arriving at an antenna array to localize a transmitter. Multipath signals arriving at the antenna array are compared to a database of calibrated multipath signal signatures and corresponding locations. The location whose calibrated signal signature best matches the measured signature is selected as the most likely transmitter location. “RF fingerprinting” typically locates a mobile transmitter with respect to a network of receivers through which the mobile transmitter travels. This type of RF fingerprinting relies on the hope that the multi-path environment will be sufficiently stable and static to be repeatable. Unfortunately, multi-path environments are often unstable and dynamic, making location determination repeatability difficult in this type of system.
Improvements in radio location systems continue to be desirable including those systems that may operate in a complex propagation environment.